const calcXY = (nums: number[]): number[] => {
  return nums.reduce(
    (result, num, i) => {
      if (i % 2 === 0) {
        result[0] += num
      } else {
        const check = num * 2
        result[1] += check > 9 ? check - 9 : check
      }
      return result
    },
    [0, 0]
  )
}

const calcT = (input: string): number => {
  const [x, y] = calcXY(input.split('').map(char => parseInt(char, 10)))
  return (x + y) % 10
}

const fakeVat = (
  input = Math.random()
    .toString(10)
    .slice(-10)
): string => {
  if (input.length !== 10) throw new Error('10 chars string only')
  return input + ((10 - calcT(input)) % 10)
}

const verifyVat = (input: string): boolean => input.length === 11 && calcT(input) === 0

export { fakeVat, verifyVat }